Accurate reception of high-speed data signals transmitted over a band-limited channel with unknown transmission characteristics requires the use of an adaptive or automatic equalizer. Such an equalizer is generally resident in the receiver portion of a data set, or modem, and is typically in the form of a transversal filter. Samples of the received data signal, hereinafter referred to as line samples, are formed at some predetermined sampling rate. The line samples are applied to the transversal filter, in which each is multiplied by a respective one of a set of coefficients. The resulting products are added together and, if necessary, demodulated to generate a baseband signal, referred to herein as an equalizer output. The value of each equalizer output is used as the basis for forming a decision as to the value of a respective transmitted data symbol.
In automatic and adaptive equalizers, the coefficients are updated or adapted using an error signal in such a way as to minimize a measure of the channel-induced distortion--assumed to be primarily intersymbol interference--in the equalizer outputs.
An important equalizer operating parameter, in addition to the rate at which the line samples are formed, is their time occurrence with respect to the received signal. On the one hand, the coefficient values subsisting in the equalizer at any given time are determined with the received signal having been sampled at a particular set of time points on the received signal. On the other hand, the transmitter and receiver clocks invariably differ from one another, if only by a small amount. Over time, this frequency difference, if not compensated for, would cause the received signal to be sampled further and further away from the appropriate time points, this phenomenon being referred to as "timing drift".
To deal with this problem, the receiver is typically provided with a timing recovery circuit. The timing recovery circuit determines whether the line samples are being formed later or earlier than they should be and, in response, adjusts the phase of the line sample forming circuit such that the line samples are formed at the appropriate time.
A variety of timing recovery circuits are known. See, for example, the so-called envelope-derived timing recovery, disclosed in The Bell System Technical Journal, Vol. 54, Page 564 et seq., March 1975, or that disclosed in U.S. Pat. No. 4,004,226 to S. U. H. Qureshi et al., or that disclosed in U.S. Pat. No. 4,334,313, issued Jun. 8, 1982 to R. D. Gitlin et al.
Although the above timing recovery schemes work satisfactorily, they are not amenable to quantitative analysis, making it difficult to "fine-tune" the timing recovery process. Such fine-tuning has become increasingly necessary with higher and higher data speeds.
More recently, in U.S. Pat. No. 4,815,103 issued to the inventors herein, a timing recovery technique is disclosed which utilizes the equalizer coefficients as the basis of timing recovery, using a computed quantity parameter referred to as the "center of gravity" of the equalizer coefficients. While this technique has provided fine-tuning capability, it is not amenable for use with a passband equalizer or multiple baseband equalizers. It would, therefore, be desirable if a timing recovery scheme possessing the advantages of the center of gravity approach could be devised which would be operable with one or more passband or baseband equalizers.